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Subject: Re: Exposure to the right and tone placement





--The high order digital bit does not contain half of the available tones.





The high order digital bit represents half of the tones that the camera
is capable of recording. Because the cameras behave as noted in the previous
paragraph—they assign counts directly proportional to photons—the high
order bit represents the brightest f-stop of data that the camera can
record.




Bruce,

I would agree that the brightest f/stop of the raw image contains half the tones as you (and Thomas Knoll) suggest and this is the rationale for ETTR. However, when the gamma curve is applied, the proportion of levels in the brightest stop is reduced to 27%:



Therefore, "underexposure" does not lose as many tones in the final picture as suggested by the 50% figure given above and no one is suggesting that this degree of underexposure is acceptable. Furthermore, the human eye can not differentiate that many tones (Weber-Fechner law). In this light, Rags' comment has some validity.

However, the original thrust of this thread was that it does not hurt to leave a little headroom at the top. Highlight tones can also be lost by clipping and non-linearity; ACR's highlight recovery can go only so far in mitigating this loss. Despite severe criticism, I think that my original post has some validity.

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